A chlorine-nitrogen co-coordinated single-atom zirconium adsorbent and its application in selective fluoride removal in high-salinity wastewater

Abstract

The present invention discloses a chlorine-nitrogen co-coordinated single-atom zirconium adsorbent and its application in selective fluoride removal in high-salt wastewater, belonging to the technical field of fluoride-containing wastewater treatment. The present invention self-assembles melamine and cyanuric acid to obtain a supramolecule, and then ball-mills the supramolecule with zirconium chloride and pyrolyzes it twice to obtain a chlorine-nitrogen co-coordinated single-atom zirconium adsorbent. The adsorbent selectively captures low-concentration fluoride ions in high-salt wastewater by applying a strong electrostatic repulsion to oxygen-containing acid radicals by negatively charged nitrogen ligands and a strong coordination effect to fluoride ions by positively charged zirconium atoms. The adsorbent prepared by the present invention not only has the advantages of high fluoride removal capacity and strong salt tolerance compared with traditional adsorbents, but also can overcome the defects of conventional chemical precipitation, coagulation and flocculation methods such as large dosage of reagents, high hazardous waste output, and high treatment cost, and has broad application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of fluorine-containing wastewater treatment, and in particular relates to a chlorine-nitrogen co-coordinated single-atom zirconium adsorbent and its application in selectively removing fluorine in high-salt wastewater. Background Art

[0002] Fluorine is widely used in industrial production processes such as glass manufacturing, electroplating etching, electronic processing, pesticide synthesis, etc., and at the same time, a large amount of fluorine-containing wastewater is generated. In actual fluorine-containing wastewater, fluoride ions (F - ) often coexist with high concentrations of inorganic salt anions such as nitrate, sulfate, and chloride. Currently, the treatment methods for fluoride-containing wastewater mainly include chemical precipitation, coagulation / flocculation, etc. Chemical precipitation method makes high concentration of F - Converted into calcium fluoride precipitate, however, it is affected by the solubility product of calcium fluoride (K sp =3.9×10 -11 ) limitation, this method can only be used to - The concentration dropped to about 10 mg / L. Although the coagulation / flocculation method can further remove F - However, the coexistence of high concentrations of inorganic salt anions in water will seriously interfere with the coagulation / flocculant effect on F - The electrostatic attraction, complexation, and netting effects of F - The concentration is reduced to meet the requirements of GB 3838-2002, "Surface Water Environmental Quality Standard." However, this method faces application bottlenecks such as the large footprint of floc settling tanks and the high amount of fluorine-containing hazardous waste generated. Therefore, the development of efficient and deep fluoride removal technology is urgently needed.

[0003] The deep fluoride removal technologies reported so far mainly include membrane separation, electrodialysis, ion exchange, adsorption, etc. Among them, adsorption has attracted much attention due to its advantages such as simple operation and small footprint. Activated alumina, magnetic iron oxide, layered double hydroxide, metal organic framework and other materials have been used for adsorption and removal of F in water. - However, under the strong site competition interference of high concentration inorganic salt anions, the defluorination performance of most adsorbents will be inhibited to varying degrees, showing the defect of poor salt tolerance. For example, Jianguo Cai et al. used polystyrene anion exchange resin to load Li / Al layered double hydroxide to prepare a defluorination adsorbent. When the adsorbent dosage was 0.5 g / L, F - When the concentration is 20 mg / L, only 40 mg / L of SO4 is added 2- or PO4 3- F -The adsorption capacity of zirconium decreased by about 67% and 71% respectively (data source: Jianguo Cai, Yanyang Zhang, Bingcai Pan, Weiming Zhang, Lu Lv, Quanxing Zhang, Efficient defluoridation of water using reusable nanocrystalline layered double hydroxides impregnated polystyrene anion exchanger, Water Research, 2016, 102, 109-116). Based on the theory of hard and soft acids and bases, the adsorption capacity of zirconium on F - Due to its strong coordination ability, zirconium-containing adsorbents have been extensively developed to enhance fluoride removal. However, currently, zirconium-containing adsorbents are not very effective in removing low-concentration F from high-salinity wastewater. - There are still two major problems: (1) The zirconium in the adsorbent has been reported to exist in the form of nanoparticles or larger, which can only expose the surface zirconium atoms for F - Most of the zirconium atoms are buried inside the particles and cannot play a role, resulting in low zirconium atomic utilization; (2) Only relying on zirconium to F - The high affinity of can only resist the interference of low concentration inorganic salts, and it is still difficult to break the strong site competition effect of high concentration inorganic salts. For example, Bingcai Pan et al. used porous polymer loaded hydrated zirconium oxide nanoparticles to prepare adsorbents in the removal of F - When (adsorbent dosage: 1.0g / L, F - concentration: 10 mg / L), 500 mg / L Cl - 、NO3 - or SO4 2- F - The adsorption capacity was reduced by over 70% (data source: Bingcai Pan, Jingsheng Xu, Bing Wu, Zhigang Li, Xitong Liu, Enhanced removal of fluoride by polystyrene anion exchanger supported hydrous zirconium oxide nanoparticles, Environmental Science & Technology, 2013, 47, 9347-9354). Therefore, how to design the structure of zirconium-containing adsorbents to fully utilize zirconium atoms and achieve selective fluoride removal from high-salinity wastewater has become a difficult problem that needs to be solved urgently. Summary of the Invention

[0004] The first technical problem to be solved by the present invention is to provide a method for preparing a chlorine-nitrogen co-coordinated single-atom zirconium adsorbent. The method pyrolyzes a mixture of melamine-cyanuric acid supramolecules and zirconium chloride twice. The second pyrolysis can further exfoliate the carbon nitride support, thereby exposing more adsorption sites, and can improve the fluorine removal capacity of the adsorbent compared to only one pyrolysis. The second technical problem to be solved by the present invention is to provide a chlorine-nitrogen co-coordinated single-atom zirconium adsorbent. The adsorbent has a large zirconium loading capacity, and the negatively charged nitrogen ligands in the single-atom zirconium sites can exert strong electrostatic repulsion to inhibit the competitive adsorption of oxygen-containing acid anions. At the same time, the chlorine-nitrogen co-coordinated structure can increase the positive charge of zirconium to increase the zirconium's absorption of F. - The third technical problem to be solved by the present invention is to provide a method for removing low concentration F from high-salt wastewater by using a chlorine-nitrogen co-coordinated single-atom zirconium adsorbent. - application.

[0005] In order to solve the above technical problems, the technical solutions adopted by the present invention are as follows:

[0006] A method for preparing a chlorine-nitrogen co-coordinated single-atom zirconium adsorbent comprises the following steps:

[0007] 1) Melamine and cyanuric acid are dissolved in hot water, respectively, and the two solutions are mixed and heated and stirred to perform a self-assembly reaction, followed by filtration and drying to obtain supramolecules;

[0008] 2) The supramolecule obtained in step 1) is mixed with zirconium chloride and ball-milled. The resulting mixed powder is placed in a tube furnace and subjected to a first pyrolysis under an inert gas protection. After cooling, the resulting powder is placed back in the tube furnace and subjected to a second pyrolysis under an inert gas protection to obtain a chlorine-nitrogen co-coordinated single-atom zirconium adsorbent.

[0009] Furthermore, in step 1), the molar ratio of melamine to cyanuric acid is 1:1; the concentration of the melamine or cyanuric acid solution is 0.1-5 mol / L; the temperature of the hot water when dissolving melamine or cyanuric acid is 80-100°C; the heating temperature when mixing the melamine and cyanuric acid solutions for self-assembly is 80-100°C, and the stirring time is 1-4 h.

[0010] Furthermore, in step 2), the mass ratio of the supramolecule to zirconium chloride is 1:0.2~1:1.2; the rotation speed of the mixed ball milling of the supramolecule and zirconium chloride is 400~800 r / min, and the ball milling time is 10~60 min.

[0011] Furthermore, in step 2), the protective gas during the first pyrolysis is nitrogen or argon, the heating rate is 1-10 °C / min, the pyrolysis temperature is 500-800 °C, and the pyrolysis time is 1-4 h.

[0012] Furthermore, in step 2), the protective gas during the second pyrolysis is nitrogen or argon, the heating rate is 5-10 °C / min, the pyrolysis temperature is 500-800 °C, and the pyrolysis time is 1-2 h.

[0013] Furthermore, the method for preparing the chlorine-nitrogen co-coordinated single-atom zirconium adsorbent can prepare the chlorine-nitrogen co-coordinated single-atom zirconium adsorbent.

[0014] Furthermore, the chlorine-nitrogen co-coordinated single-atom zirconium adsorbent can remove low-concentration F - application.

[0015] Furthermore, F - The concentration is not higher than 10 mg / L.

[0016] Furthermore, F - When the concentration is not higher than 10 mg / L, the dosage of adsorbent in wastewater is 0.25 ~ 0.50 g / L.

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] (1) There is no report on the prior art regarding the chlorine-nitrogen co-coordinated single-atom zirconium adsorbent and its application in defluorination. The adsorbent of the present invention can not only utilize the unsaturated coordination structure of the zirconium atom, but also give full play to its ability to remove fluorine. - The specific adsorption capacity of zirconium can be enhanced by the presence of chlorine and nitrogen ligands, especially axial chlorine ligands, which can increase the positive charge of zirconium and increase the absorption of single-atom zirconium sites to F. - The binding strength is very good, so it is suitable for selective fluoride removal in high-salt wastewater.

[0019] (2) In the zirconium-based adsorbents available on the market or reported in the literature, zirconium mainly exists in the form of nanoparticles or larger sizes. The zirconium atoms on the surface of the particles are actually used to remove fluorine. Most of the zirconium is buried inside the particles and cannot react with F. - The zirconium in the adsorbent prepared by the present invention is dispersed in the form of single atoms on the two-dimensional carbon nitride support, and all zirconium atoms can be exposed as F - The adsorption sites have a theoretical utilization rate of 100% of zirconium atoms, which can significantly improve the atomic economy of the adsorbent and reduce the preparation cost.

[0020] (3) Compared with conventional adsorbents, it can remove low concentration F in high-salt wastewater. - The fluorine removal efficiency is greatly attenuated due to the site competition effect. The chlorine-nitrogen co-coordinated single-atom zirconium site in the adsorbent of the present invention is very effective for F -It has unique selectivity. The zirconium in this site is positively charged, while chlorine and nitrogen are negatively charged. When applied to water, the chloride ions will first be ionized and enter the water, thereby exposing the single-atom Zr-N site. When oxygen-containing acid radicals such as sulfate and nitrate are close to the Zr-N site, the negatively charged N ligand will exert a strong electrostatic repulsion on the negatively charged O in the oxygen-containing acid radical, making it difficult for the oxygen-containing acid radical to stably bind to Zr and be adsorbed; when chloride ions or F - When anions containing only one atom and no oxygen are close to the Zr-N site at the same time, Zr reacts with F - Stronger coordination affinity leads to F - Therefore, the monoatomic zirconium adsorbent prepared by the present invention can selectively adsorb low concentration F in high-salt wastewater. - Even if the concentration of coexisting inorganic salt anions is as high as 100 mmol / L (10 mg / LF - 190 times the concentration), can still achieve F - Efficient removal.

[0021] (4) The carbon nitride carrier prepared by the present invention has intrinsic nanopores and rich nitrogen ligands, which are suitable for high-load anchoring of zirconium atoms. The zirconium loading in the optimal single-atom zirconium adsorbent prepared is as high as 35.66 wt%, and its fluorine removal capacity is 5.6 times and 21.5 times that of the commercially available anion exchange resin D213 resin and zirconium dioxide nanoparticles, respectively. In practical application, it can greatly reduce the amount of adsorbent added and the size of the adsorption device. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a spherical aberration corrected scanning transmission electron micrograph of the CNZr2-6 adsorbent prepared in Example 1 of the present application;

[0023] Figure 2 High-angle annular dark field (HAADF)-scanning transmission electron microscopy image and elemental surface scanning image of the CNZr2-6 adsorbent prepared in Example 1 of the present application;

[0024] Figure 3 This is the X-ray absorption fine structure wavelet transform image of the CNZr2-6 adsorbent prepared in Example 1 of the present application;

[0025] Figure 4 This is a comparison chart of the fluorine removal performance of the CNZr1-6 adsorbent and the CNZr2-6 adsorbent prepared in Example 1 of the present application;

[0026] Figure 5 This is a comparison chart of the fluorine removal performance of adsorbents synthesized from three different carbon nitride precursors in Example 1 and Comparative Examples 1-2 of the present application;

[0027] Figure 6This is a comparison chart of the fluorine removal performance of the adsorbent synthesized at different zirconium chloride dosages in Example 2 of the present application;

[0028] Figure 7 This is a comparison chart of the fluorine removal performance of the adsorbents synthesized at different pyrolysis temperatures in Example 3 of the present application;

[0029] Figure 8 The figures show the effects of different concentrations and types of inorganic salt anions on the fluoride removal performance of the CNZr2-6 adsorbent prepared in Example 1 of the present application; wherein, in Figure (a), the adsorbent dosage is 0.25 g / L; in Figure (b), the adsorbent dosage is 0.35 g / L; and in Figure (c), the adsorbent dosage is 0.50 g / L.

[0030] Figure 9 This is a comparison chart of the fluorine removal performance of PAMD resin, D213 resin and ZrO2 nanoparticles in Comparative Example 3 of this application. DETAILED DESCRIPTION

[0031] The present invention will be further illustrated below with reference to specific examples. The examples are implemented based on the technical solutions of the present invention. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention.

[0032] Melamine, cyanuric acid, zirconium chloride, and fluoroboric acid used in the following examples were all analytically pure and purchased from Aladdin Reagent Co., Ltd.; sodium fluoride, sodium hydroxide, sodium chloride, sodium citrate dihydrate, acetic acid, nitric acid, and sulfuric acid were all analytically pure and purchased from Sinopharm Chemical Reagent Co., Ltd.; hydrochloric acid was analytically pure and purchased from Nanjing Chemical Reagent Co., Ltd.; PAMD resin is an acrylic resin grafted with triethylenetetramine, with patent number ZL 201310108031.4; D213 resin is a quaternary ammonium salt-modified styrene resin purchased from Tianjin Yunkai Resin Technology Co., Ltd.; ZrO2 nanoparticles with a particle size of 20~40nm and a purity of 99 wt% were purchased from Jiangsu Xianfeng Nanomaterial Technology Co., Ltd.

[0033] The thermal digestion procedure in the following examples is as follows: 5 mg of the adsorbent is weighed into a polytetrafluoroethylene digestion tube, 2 mL of nitric acid, 2 mL of sulfuric acid, and 2 mL of fluoroboric acid are added in sequence, and then the digestion procedure is started. First, pre-digestion is performed at 60°C for 30 min, then the temperature is increased to 160°C for digestion for 30 min, and finally the temperature is increased to 300°C for digestion for 45 min. After cooling to room temperature, the volume is adjusted to 100 mL, and the zirconium content in the digestion solution is determined using an inductively coupled plasma optical emission spectrometer.

[0034] In the following examples, F -The concentration test method is as follows: take 20 mL of the solution after adsorption equilibrium, filter it with a 0.22 μm filter head, add 10 mL of total ionic strength adjustment buffer (TISAB) to the filtrate, dilute it to 50 mL, transfer it to a 100 mL polyethylene beaker, and use a fluoride ion meter to measure the F in the solution. - concentration.

[0035] The total ionic strength adjustment buffer (TISAB) is prepared as follows: weigh or measure 58 g of sodium chloride, 10 g of sodium citrate dihydrate, and 50 mL of acetic acid into 500 mL of water, mix well, add 135 mL of 5 mol / L sodium hydroxide solution, adjust the solution pH to 5.2, and dilute to 1 L with water.

[0036] Example 1

[0037] A method for preparing a chlorine-nitrogen co-coordinated single-atom zirconium adsorbent comprises the following steps:

[0038] 1) Supramolecular synthesis

[0039] 0.05 mol of melamine was added to 50 mL of ultrapure water and heated with stirring at 80°C until completely dissolved to obtain solution A. 0.05 mol of cyanuric acid was added to 50 mL of ultrapure water and heated with stirring at 80°C until completely dissolved to obtain solution B. Solution B was poured into solution A and heated with stirring at 80°C for 4 h. After the self-assembly of melamine and cyanuric acid was completed, the mixture was separated by filtration and dried in an oven at 60°C to obtain the supramolecule.

[0040] 2) Synthesis of chlorine-nitrogen co-coordinated single-atom zirconium adsorbent

[0041] 2 g of supramolecules were mixed with 6 mmol of ZrCl4 and ball milled at 550 r / min for 15 min. The resulting mixture was placed in a tubular furnace and heated to 600 °C at a rate of 5 °C / min in a nitrogen atmosphere and maintained for 2 h to obtain a primary pyrolysis material (named CNZr1-6). The material obtained by the primary pyrolysis was placed in a tubular furnace and heated to 600 °C at a rate of 10 °C / min in a nitrogen atmosphere and maintained for 1 h. After this secondary pyrolysis, a chlorine-nitrogen co-coordinated single-atom zirconium adsorbent (named CNZr2-6) was obtained.

[0042] Figure 1 This is a spherical aberration corrected scanning transmission electron microscopy image of the CNZr2-6 adsorbent, showing high-density zirconium single atoms dispersed on the porous carbon nitride support.

[0043] Figure 2This is the high-angle annular dark field (HAADF)-scanning transmission electron microscopy image and element surface scanning image of CNZr2-6 adsorbent, which shows that the adsorbent contains C, N, Zr, and Cl elements.

[0044] Figure 3 This is the X-ray absorption fine structure wavelet transform image of CNZr2-6 adsorbent. It can be seen that Zr-N and Zr-Cl coordination bonds exist simultaneously in CNZr2-6 adsorbent, that is, the zirconium atom is a chlorine-nitrogen co-coordination structure.

[0045] The defluoridation experiment of water was carried out using CNZr1-6 adsorbent and CNZr2-6 adsorbent, including the following steps: preparing the initial F using sodium fluoride as raw material; - The concentration of F was 10 mg / L - Solution, adjust the pH to 3 with dilute hydrochloric acid solution, take 40 mL F - The solution was added to a 50 mL plastic centrifuge tube. The adsorbent was then added to the centrifuge tube at a dosage of 0.25 g / L. Ultrasonication was performed for 2 minutes to uniformly disperse the adsorbent. The solution was then shaken at 25°C and 200 r / min for 48 hours to allow the adsorption to reach equilibrium. An appropriate amount of the solution was filtered through a 0.22 μm aqueous polytetrafluoroethylene filter. 10 mL of the filtrate was placed in a 50 mL colorimetric tube. 10 mL of total ionic strength adjustment buffer (TISAB) was accurately added. The solution was diluted to the mark with pure water, shaken, and poured into a 100 mL polyethylene beaker. The fluoride content of the solution at equilibrium was determined using a fluoride ion meter. - concentration.

[0046] Figure 4 The comparison of fluorine removal performance of CNZr1-6 adsorbent and CNZr2-6 adsorbent shows that secondary pyrolysis can further peel off the adsorbent, thereby exposing more adsorption sites and removing F - The adsorption capacity increased from 32.85 mg / g to 35.02 mg / g.

[0047] Comparative Example 1

[0048] 2 g of melamine (MA) was mixed with 6 mmol of ZrCl4 and ball milled at 550 r / min for 15 min. The resulting mixture was placed in a tube furnace and heated to 600 °C at a rate of 5 °C / min in a nitrogen atmosphere and maintained for 2 h to obtain a primary pyrolysis material. The material obtained by the primary pyrolysis was placed in a tube furnace and heated to 600 °C at a rate of 10 °C / min in a nitrogen atmosphere and maintained for 1 h. After this secondary pyrolysis, a chlorine-nitrogen co-coordinated single-atom zirconium adsorbent (named CNZr2-6-MA) was obtained.

[0049] Comparative Example 2

[0050] 2 g of urea (Urea) was mixed with 6 mmol of ZrCl4 and ball milled at 550 r / min for 15 min. The resulting mixture was placed in a tube furnace and heated to 600 °C at a rate of 5 °C / min in a nitrogen atmosphere and maintained for 2 h to obtain a primary pyrolysis material; the material obtained by the primary pyrolysis was placed in a tube furnace and heated to 600 °C at a rate of 10 °C / min in a nitrogen atmosphere and maintained for 1 h. After this secondary pyrolysis, a chlorine-nitrogen co-coordinated single-atom zirconium adsorbent (named CNZr2-6-Urea) was obtained.

[0051] The adsorbents prepared in Comparative Examples 1 and 2 were respectively tested for fluoride removal performance in water, using the same testing method as in Example 1. Figure 5 The fluorine removal performance comparison chart of adsorbents synthesized from three different carbon nitride precursors, CNZr2-6, CNZr2-6-MA and CNZr2-6-Urea, shows that the adsorbent CNZr2-6 prepared with supramolecule as precursor has the best fluorine removal effect, and the adsorption capacity (Q e ) reached 35.02 mg / g. Therefore, the supramolecule was selected as the best precursor for the preparation of fluorine removal adsorbent.

[0052] Example 2

[0053] When preparing chlorine-nitrogen co-coordinated single-atom zirconium adsorbents, the amount of ZrCl4 used in step 2) was replaced by 6 mmol to 0, 2, 4, 8 or 10 mmol. The rest of the preparation method and parameters were the same as in Example 1. The obtained adsorbents were respectively recorded as CNZr2-0, CNZr2-2, CNZr2-4, CNZr2-8, and CNZr2-10.

[0054] The adsorbents prepared in Example 2 were tested for fluoride removal performance in water using the same testing methods as in Example 1. Figure 6 The comparison chart of the defluorination performance of the synthesized adsorbents under different ZrCl4 dosages shows that with the increase of ZrCl4 dosage, the defluorination performance of the obtained adsorbents is better. - The adsorption capacity of the adsorbent first gradually increased, reaching a maximum of 35.02 mg / g when the ZrCl4 dosage was 6 mmol; when the ZrCl4 dosage was further increased, the fluorine removal performance of the adsorbent began to gradually decrease. In addition, when the ZrCl4 dosage was 0, the adsorbent CNZr2-0 had a low adsorption capacity for F. - The adsorption amount is 0, which shows that zirconium plays a leading role in fluorine removal.

[0055] CNZr2-x (x = 2, 4, 6, 8, 10) was digested by thermal digestion, and the zirconium content in the digestion solution was then determined using inductively coupled plasma optical emission spectrometry. The mass fraction of zirconium in each adsorbent was calculated based on the measured results. The results are detailed in Table 1.

[0056] Table 1 Mass fraction of zirconium in CNZr2-x adsorbents

[0057]

[0058] From the results in Table 1, it can be seen that with the increase of ZrCl4 dosage, the mass fraction of zirconium in the obtained adsorbent also gradually increases. Figure 6 It can be seen that when the ZrCl4 dosage increases to 8 and 10 mmol, the fluorine removal performance of the resulting adsorbent decreases. This is because when the zirconium mass fraction is too high, the carbon nitride support cannot provide sufficient nitrogen ligands to anchor all zirconium atoms. Some zirconium atoms begin to aggregate to form ZrO2 nanoparticles, thereby reducing the fluorine removal efficiency. Therefore, the optimal ZrCl4 dosage is 6 mmol, at which the mass ratio of supramolecule to ZrCl4 is 1:0.70.

[0059] Example 3

[0060] When preparing the chlorine-nitrogen co-coordinated single-atom zirconium adsorbent, the pyrolysis temperature in step 2) was changed from 600°C to 500°C, 700°C or 800°C, and the remaining preparation methods and parameters were the same as in Example 1.

[0061] The adsorbents prepared in Example 3 were tested for fluoride removal performance in water using the same testing methods as in Example 1. Figure 7 The figure is a comparison of the fluorine removal performance of the adsorbents synthesized at different pyrolysis temperatures. It can be seen that as the pyrolysis temperature increases, the obtained adsorbent has a better defluorination performance. - The adsorption capacity first increases and then decreases, reaching a maximum of 35.02 mg / g at 600°C. When the calcination temperature increases from 500°C to 600°C, a large amount of gas is released from the supramolecular precursor, creating high pressure between the carbon nitride layers, which promotes exfoliation. The carbon nitride acts as a porogen, creating abundant pores within the carbon nitride nanosheets, increasing the adsorbent's specific surface area and enhancing the exposure of adsorption sites, thereby significantly enhancing fluorine removal performance. However, when the pyrolysis temperature continues to increase from 600°C to 700°C and 800°C, zirconium atoms, driven by thermal energy, break free from the carbon nitride support and combine with oxygen atoms in the supramolecular precursor, subsequently migrating and aggregating to form ZrO2 nanoparticles, significantly reducing the fluorine removal capacity to 24.92 and 11.81 mg / g, respectively. Therefore, the optimal pyrolysis temperature for preparing chlorine-nitrogen co-coordinated single-atom zirconium adsorbent is 600°C.

[0062] Example 4

[0063] Five common inorganic salt anions in industrial wastewater were selected as F - Coexisting interfering ions, including SiO3 2- PO4 3- 、SO4 2- 、NO3- and Cl - , and selected three adsorbent dosages (0.25 g / L, 0.35 g / L, 0.50 g / L) to further study the adsorbent CNZr2-6 synthesized in Example 1 to remove low concentration F under the interference of high concentration inorganic salts. - The specific operation steps are: use any one of sodium metasilicate nine hydrate, sodium dihydrogen phosphate, sodium sulfate, sodium nitrate or sodium chloride and sodium fluoride as raw materials, and prepare a series of F-containing - and mixed solutions of inorganic salt anions with different concentrations, where F - The concentration was controlled at 10 mg / L, and the concentrations of the coexisting inorganic anions were set at 0, 5, 10, 20, 50, and 100 mmol / L, respectively. The pH of the mixed solution was adjusted to 3 with hydrochloric acid solution. 40 mL of the mixed solution was added to a 50 mL plastic centrifuge tube. CNZr2-6 adsorbent was then added to the centrifuge tube at dosages of 0.25 g / L, 0.35 g / L, or 0.50 g / L. Ultrasonication was performed for 2 minutes to uniformly disperse the adsorbent. The tube was then shaken at 25°C and 200 rpm for 48 hours to allow adsorption equilibrium. An appropriate amount of the solution was filtered through a 0.22 μm aqueous polytetrafluoroethylene filter. 10 mL of the filtrate was placed in a 50 mL colorimetric tube, and 10 mL of total ionic strength adjustment buffer (TISAB) was accurately added. The solution was diluted to the mark with pure water, shaken, and poured into a 100 mL polyethylene beaker. The fluoride content in the solution at equilibrium was determined using a fluoride ion meter. - concentration.

[0064] Figure 8 The results show that CNZr2-6 adsorbent can remove low concentration F in the presence of different types and concentrations of inorganic anions. - When the adsorbent dosage is low (0.25 g / L), high concentration of coexisting anions and low concentration of F - The strong competition for limited adsorption sites resulted in a decrease in the fluoride removal capacity of CNZr2-6 compared to the absence of anions. Increasing the adsorbent dosage can provide more adsorption sites, which is beneficial to improving the F removal capacity under high concentration inorganic salt stress. - When the adsorbent dosage increases to 0.50 g / L, even if the inorganic salt anion SiO3 2- PO4 3- 、SO4 2- 、NO3 - and Cl - Concentrations up to 100 mmol / L are F -The fluoride removal capacity of CNZr2-6 was only reduced by 9.15%, 0.66%, 2.04%, 2.25% and 0.77% respectively compared with the case without anion coexistence, showing excellent resistance to inorganic salt interference. Therefore, CNZr2-6 adsorbent is suitable for selective removal of low concentration F from high concentration inorganic salt solution. - .

[0065] Comparative Example 3

[0066] The defluorination performance of PAMD resin, D213 resin and ZrO2 nanoparticles was tested. The test method was different from that in Example 1 except that the dosage of the adsorbent was 0.50 g / L. In addition, sodium sulfate was used as a representative inorganic salt to study the salt tolerance of the three adsorbents during defluorination. - The concentration of coexisting sodium sulfate was set to 0.05 mol / L.

[0067] Figure 9 The comparison chart of fluorine removal performance of PAMD resin, D213 resin and ZrO2 nanoparticles shows that at the dosage of 0.50 g / L, PAMD resin, D213 resin and ZrO2 nanoparticles have the best fluorine removal performance. - The adsorption capacity of CNZr2-6 adsorbent for F was only 1.68, 6.30 and 1.63 mg / g respectively. - The adsorption capacity of CNZr2-6 was 19.57 mg / g, which was 11.65 times, 3.11 times, and 12.01 times that of PAMD resin, D213 resin, and ZrO2 nanoparticles, respectively. More importantly, under the interference of 0.05 mol / L sodium sulfate, the fluoride removal capacity of PAMD resin, D213 resin, and ZrO2 nanoparticles decreased to 0.28, 0, and 0.32 mg / g, respectively, indicating that its salt tolerance was significantly weaker than that of CNZr2-6.

[0068] Application Examples

[0069] In order to explore the performance of chlorine-nitrogen co-coordinated single-atom zirconium adsorbent in treating actual high-salt fluoride-containing wastewater, the production wastewater of a photovoltaic enterprise in Changzhou, Jiangsu was selected for the experiment. The water quality of the wastewater is shown in Table 2 below.

[0070] Table 2 Water quality of production wastewater from a photovoltaic enterprise in Changzhou, Jiangsu

[0071]

[0072] A fluorine removal experiment was conducted using the CNZr2-6 adsorbent synthesized in Example 1, comprising the following steps: adjusting the pH of the fluorine-containing industrial wastewater to 3 using a hydrochloric acid solution, then adding 40 mL of the wastewater to a 50 mL plastic centrifuge tube, adding the adsorbent at a dosage of 0.50 g / L to the centrifuge tube, ultrasonicating for 2 minutes to uniformly disperse the adsorbent, and then oscillating at a constant temperature of 25°C and 200 r / min for 48 hours to allow adsorption to reach equilibrium. An appropriate amount of the solution was filtered using a 0.22 μm aqueous polytetrafluoroethylene filter membrane, 10 mL of the filtrate was placed in a 50 mL colorimetric tube, 10 mL of total ionic strength adjustment buffer (TISAB) was accurately added, the solution was diluted to the mark with pure water, shaken well, and poured into a 100 mL polyethylene beaker. The F in the solution at equilibrium was determined using a fluoride ion meter. - Concentration. The experimental results show that 95.27% of F in industrial wastewater - was removed, showing the excellent performance of chlorine-nitrogen co-coordinated single-atom zirconium adsorbent in treating actual high-salt fluoride-containing wastewater.

[0073] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing a chlorine-nitrogen co-coordinated single-atom zirconium adsorbent, characterized by: The following steps are involved: 1) Melamine and cyanuric acid are dissolved in hot water, respectively, and the two solutions are mixed and heated and stirred to perform a self-assembly reaction, followed by filtration and drying to obtain supramolecules; 2) mixing the supramolecule obtained in step 1) with zirconium chloride and ball milling the resulting mixed powder, placing the resulting mixed powder in a tube furnace and performing a first pyrolysis under inert gas protection, and after cooling, returning the resulting powder to the tube furnace and performing a second pyrolysis under inert gas protection to obtain a chlorine-nitrogen co-coordinated single-atom zirconium adsorbent; wherein the mass ratio of the supramolecule to zirconium chloride is 1:0.2 to 1:1.2, the first pyrolysis temperature is 500°C to 800°C, and the second pyrolysis temperature is 500°C to 800°C.

2. The method for preparing the chlorine-nitrogen co-coordinated single-atom zirconium adsorbent according to claim 1, characterized in that: In step 1), the molar ratio of melamine to cyanuric acid is 1:1; the concentration of the melamine or cyanuric acid solution is 0.1 to 5 mol / L; the temperature of the hot water for dissolving the melamine or cyanuric acid is 80 to 100°C; the heating temperature for self-assembly after mixing the melamine and cyanuric acid solutions is 80 to 100°C, and the stirring time is 1 to 4 hours.

3. The method for preparing the chlorine-nitrogen co-coordinated single-atom zirconium adsorbent according to claim 1, characterized in that: In step 2), the speed of ball milling the supramolecule and zirconium chloride is 400-800 r / min, and the ball milling time is 10-60 min.

4. The method for preparing the chlorine-nitrogen co-coordinated single-atom zirconium adsorbent according to claim 1, characterized in that: In step 2), the protective gas during the first pyrolysis is nitrogen or argon, the heating rate is 1-10 °C / min, and the pyrolysis time is 1-4 h.

5. The method for preparing the chlorine-nitrogen co-coordinated single-atom zirconium adsorbent according to claim 1, characterized in that: In step 2), the protective gas during the second pyrolysis is nitrogen or argon, the heating rate is 5-10 °C / min, and the pyrolysis time is 1-2 h.

6. The chlorine-nitrogen co-coordinated single-atom zirconium adsorbent prepared according to the method for preparing a chlorine-nitrogen co-coordinated single-atom zirconium adsorbent according to any one of claims 1 to 5.

7. Use of the chlorine-nitrogen co-coordinated single-atom zirconium adsorbent according to claim 6 for removing low-concentration fluoride ions in high-salinity wastewater.

8. Use of the chlorine-nitrogen co-coordinated single-atom zirconium adsorbent according to claim 7 for removing low-concentration fluoride ions in high-salinity wastewater, characterized in that: The fluoride ion concentration is not higher than 10 mg / L.

9. Use of the chlorine-nitrogen co-coordinated single-atom zirconium adsorbent according to claim 8 for removing low-concentration fluoride ions in high-salinity wastewater, characterized in that: The dosage of adsorbent in wastewater is 0.25 ~ 0.50 g / L.

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